Update documentation organisation and landing page #3541

Job	Run time
cppcheck	5m 19s
clang-format	18s
black_IGNORED	30s
copyright_headers	15s
flake8	18s
forbidden_types	11s
lychee_IGNORED	19s
mypy	32s
pydocstyle	21s
pylint	45s
rstcheck	16s
unused_names	15s
vale	39s
sphinx-rtd	3m 1s
sphinx-conda	0s
build_linux (ubuntu-20.04, gcc, boost, optimize, warning)	12m 51s
build_linux (ubuntu-20.04, gcc, openmp, python, gsl, ltdl, boost, optimize, warning)	19m 4s
build_linux (ubuntu-20.04, gcc, mpi, python, gsl, ltdl, boost, optimize, warning)	19m 24s
build_linux (ubuntu-20.04, gcc, openmp, mpi, python, gsl, ltdl, boost, hdf5, sionlib, libneurosim...	32m 40s
build_macos (macos-latest, clang, openmp, mpi, python, gsl, ltdl, boost, hdf5, optimize, warning)	32m 59s
	2h 9m 57s

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